Bilayer structure of ganglioside/cholesterol mixed system in the presence of Ca 2+

We employed small‐angle X‐ray scattering measurement using synchrotron radiation to study structural properties of monosialoganglioside (G M1 )/cholesterol (chol) mixtures in aqueous dispersions, in the presence of 0–50 m M CaCl 2 . Various molar ratios of G M1 /chol were examined from 1:0 to 1:5. Without CaCl 2 the radius of gyration ( R g ) of the aggregate increased linearly with increasing cholesterol content up to G M1 /chol = 1/1. At cholesterol content above G M1 /chol = 1/1, R g of the aggregate was almost constant, and a sharp diffraction peak at q = 0.18 Å −1 , characteristic of cholesterol monohydrate crystals, was observed, suggesting the solubility limit of cholesterol in G M1 aggregates is about G M1 /chol = 1/1. For the samples containing cholesterol of G M1 /chol = 1/0.6 and above, the diffraction patterns showed a set of peaks that correspond to ~85 Å periodic lamellar structures in the presence of CaCl 2 at concentrations higher than ~10 m M . The observed lamellar peak positions and the intensity ratio of the lamellar peaks (1st–4th) was not changed by the addition of CaCl 2 up to 100 m M , demonstrating once formed multilayered structure of G M1 /chol was not altered under the present conditions. Electron density profiles for this G M1 /chol bilayer were obtained. The Ca 2+ ‐induced vesicle‐to‐lamellar phase separation of this system was found to be highly reversible by the Ca 2+ chelation with ethylenediamine‐tetraacetic acid.